Advanced composites are high strength, high modulus materials which are finding increasing use as structural components in aircraft, automotive, and sporting goods applications. Typically they comprise structural fibers such as carbon fibers in the form of woven cloth or continuous filaments embedded in a thermosetting resin matrix.
Composite properties depend on both the matrix resin and the reinforcement. In unidirectional carbon fiber composites, important mechanical properties include longitudinal tensile strength and modulus, transverse tensile strength and modulus, and longitudinal compressive strength. The matrix affects all of these properties, but has the greatest effect on compressive strength and transverse tensile properties. High composite compressive strengths and transverse tensile moduli require that the matrix have a high modulus.
State-of-the-art epoxy matrix resin systems in advanced composites are typically based on N,N,N',N'-tetraglycidyl 4,4'-diaminodiphenyl methane and 4,4'-diaminodiphenyl sulfone. These resins produce unreinforced castings which have tensile strengths of about 8,000 psi and tensile moduli of 500,000 to 550,000 psi. Unidirectional composites containing 60 volume fraction fiber made with these matrix resins typically have transverse tensile strengths of 5,000 to 7,000 psi and transverse tensile moduli of 1.0 to 1.4 million psi. Higher transverse properties are very desirable for applications such as pressure vessels.
One reason that matrix resins containing N,N,N',N'-tetraglycidyl 4,4'-diaminodiphenyl methane and 4,4'-diaminodiphenyl sulfone are widely used in advanced composites is that they possess the balance of properties required for making prepreg, a ready-to-mold sheet of reinforcement impregrated with uncured or partially cured resin. These characteristics include: 1) a tacky, dough-like consistency, 2) low reactivity at room temperature, and 3) a high degree of cure after heating for 2 hours at 179.degree. C. in an autoclave. Many epoxy resin systems lack at least one of these characteristics, and therefore are unsuitable for prepreg. There is a need for epoxy resin systems which are prepregable and which, when used, produce unreinforced castings which have higher tensile properties than state-of-the-art prepreg resin systems.
It has been found that compositions comprising: a) an epoxy resin containing at least one glycidyl amine group, and b) a select group of monoamines simultaneously satisfy both requirements.
U.S. Pat. No. 2,951,822 discloses epoxy resins containing glycidyl amine groups which may be cured with monoamines such as aniline and m-chloroaniline. However, the patent states that in preparing thermosetting compositions, a molar proportion of from about 0.7 to 1.3 epoxy groups per NH group is used. This represents a ratio of equivalents of amine NH groups to equivalents of epoxide groups of 0.77 to 1.43.
In the present invention it has been found that in a composition containing an epoxy resin having at least one glycidyl amine group and a particular aromatic amine hardener, the ratio of equivalents of amine NH groups to equivalents of epoxide groups must be less than 0.77 (that ratio required by the prior art) to simultaneously achieve prepregability and high matrix strengths and moduli.